This invention relates to the production of metal such as aluminum from a metal chloride or other compound of the metal dissolved in molten solvent bath by electrolyzing the bath in a monopolar or bipolar electrolysis cell. More particularly, the invention relates to graphite electrodes used in such cells and to reducing their wear characteristics so as to prolong useful electrode life in such cells and to controlled methods of graphite electrode manufacture to achieve such reduced wear rates.
One type of electrolytic cell used in the production of metal, such as aluminum, from metal chloride dissolved in a solvent salt bath includes a terminal anode, at least one intermediate bipolar electrode and a terminal cathode. These electrodes are typically situated in relatively closely spaced, generally parallel relationship wherein opposed anode-cathode faces provide interelectrode spaces through which the molten bath can move and be electrolyzed by passage of current from anode to cathode. Electrolysis of the metal chloride occurring within the interelectrode space results in molten metal depositing at the cathode and chlorine gas collecting at the anode. Cells of this type are described in U.S. Pat. Nos. 3,755,099, 3,822,195 and 4,179,345, incorporated herein by reference. One of the important features of these cells is that the anode-to-cathode space or distance should be carefully maintained at a preselected level in order to achieve the lower power consumption capabilities of the bipolar chloride electrolysis process. Any amount of wear occurring on either the anode or the cathode surface, as by erosion or other removal of electrode material, tends to increase the distance and, accordingly, increase the electrical resistance across the distance between anode and cathode. For the most part, the anode presents little problem since under most conditions chlorine is relatively non-corrosive to the carbonaceous materials employed for anodes. However, experience has shown that some amount of electrode wear does occur on the cathode surface, and considerable effort has been expended for reducing or relieving this wear condition. Excessive cathode surface wear is a problem, not only respecting increased power consumption as just explained, but there is the further possibility of increasing the resistance so much that the cell is considered uneconomical to operate, thus necessitating a costly shutdown, repair or replacement of the electrodes, and restarting the cell. In addition to the electrical resistance problems resulting from cathode wear, the carbonaceous material removed from the cathode surface can contaminate the bath. This alone can reach such an extreme as to necessitate shutting down the cell.